Our invention relates to phase selective track circuit apparatus. More particularly, the invention pertains to a frequency selective filter network for use in such a track circuit to reject interfering currents of different frequencies, induced in the rails by propulsion currents, from actuating improper operation of the track relay.
Phase selective track circuits require that the phase angle between the track voltage signal and the reference or local voltage signal be within a prescribed range, for example, within plus or minus thirty degrees of opposition. The track signal unavoidably undergoes a phase shift as it is transmitted from its source to the receiving detector due to the presence of various circuit elements such as a current limiting device, impedance bonds for coupling traction power between track circuits, and the track rails and ballast. Since the amount of phase shifts depends upon ballast resistance (wet vs. dry), it is necessary to assure that the resulting change will, at all times, fall within the prescribed range. The magnitude of the phase shift is also dependent upon factors such as the track circuit length and the values at operating frequency of the impedance of the bonds, the current limiting device, and the load at the receiving end of the circuit. In early installations, phase corrections were made by connecting a capacitor, a resistor, or combinations of these in series with the detector. In multi-track applications in alternating current (AC) electrified territory, there is mutual coupling between the propulsion supply of a given track and an adjacent track which induces a circulating current in this adjacent track circuit. This inductive interference, while not of the same frequency as the track circuit signal, can become great enough under some conditions to disrupt normal operation of the track circuit. Although not unsafe, because the interfering energy is not coded as is the normal signal energy, occurrences of this type can cause undesirable false restrictive aspects to be displayed by the signals. Filters with fixed elements have been used in series with the receiving detector to provide rejection of the induced interference. The fixed reactive elements comprising such filters provide, at a fixed signaling frequency, the requirements of proper rejection and phase correction for only a limited range of track circuit lengths and ballast resistances. Outside this range or at a different signaling frequency, one or both requirements are not met. Thus, an improved filter with variable reactance is needed.
Accordingly, an object of our invention is an improved phase selective track circuit arrangement for use on alternating current electrified railroads.
Another object of the invention is phase selective track circuit apparatus having a variable frequency selective filter adjustable to apply the track circuit to various length track sections having different track and apparatus impedances.
Also, an object of our invention is a frequency selective filter for improving the operation of phase selective track circuits on electrified railroads.
Another object of the invention is a tuned filter unit which has selective external connections to enable its use in phase selective track circuits of any length.
A further object of the invention is a frequency selective filter for phase selective track circuits which includes a capacitor connected in series with two tapped reactor windings which are separated by a series resistor, the winding taps providing inductance adjustment for different length track sections and impedance conditions, the resistor being included in the filter circuit for short track circuits to provide improved signal to noise ratio where critical operating conditions may occur, a special tap being included to shift the tuninng to a second track circuit frequency.
Yet another object of our invention is a phase selective track circuit arrangement including a series L-C filter, at the receiver end, having a selectively variable inductance to match the impedance of different length track circuits to enable tuning to reject induced electric propulsion frequency currents.
A still further object of the invention is a frequency selective filter for use in phase selective track circuits and having a series L-C network with adjustable inductive reactance to tune the track circuit to reject induced currents of other frequencies regardless of the length and parameters of the track circuit.
Other objects, features, and advantages of our invention will become apparent from the following specification and appended claims, when taken in connection with the accompanying drawings.